Encapuslate UTF-8 decoder state in a struct

include/extern/utf8decoder.hpp

@@ -8,6 +8,11 @@
 #define UTF8_ACCEPT 0
 #define UTF8_REJECT 12
 
-uint32_t decode(uint32_t *state, uint32_t *codep, uint8_t byte);
+struct Utf8Decoder {
+	uint32_t state = UTF8_ACCEPT;
+	uint32_t codepoint = 0;
+
+	uint32_t update(uint8_t byte);
+};
 
 #endif // RGBDS_EXTERN_UTF8DECODER_HPP